I. Field of the Invention
The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved method and apparatus for transmitting voice and data in a code division multiple access (CDMA) wireless telecommunications system.
II. Description of the Related Art
One measure of the usefulness of a wireless communication system is the efficiency with which it uses the available RF bandwidth. In some instances, efficiency is defined as the sustainable data transmission rate of a system over a given amount of RF bandwidth. In other instances, efficiency is characterized as the total number of communications (such as telephone calls) that can be simultaneously conducted using a given amount of bandwidth. Whatever the measure, increasing efficiency generally increases the usefulness of a wireless communication system.
An example of an especially efficient, and therefore especially useful, wireless communication system is shown in FIG. 1, which is a highly simplified illustration of a wireless cellular telephone system configured in accordance with the IS-95 over-the-air interface standard adopted by the Telecommunications Industry Association (TIA). The IS-95 standard and its derivatives such as IS-95-A etc. (referred to herein collectively as the IS-95 standard) define a set of code division multiple access (CDMA) signal processing techniques for implementing a cellular telephone system. A cellular telephone system configured substantially in accordance with the IS-95 standard is described in U.S. Pat. No. 5,103,459 entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d assigned to the assignee of the present invention and incorporated herein by reference.
In accordance with the IS-95 standard, subscriber units 10a-c (usually cellular telephones) conduct telephone calls and other communications by interfacing with one or more base stations 12 using the CDMA modulated RF signals. Each interface is comprised of a forward link signal transmitted from the base station 12 to the subscriber unit 10, and a reverse link signal transmitted from the subscriber unit 10 to the base station 12.
Base station controllers (BSC) 14 perform various functions that allow mobile communication to take place including orchestrating the handoff of a subscriber unit 10 between two base stations 12. Mobile switching center (MSC) 16 provides call processing and routing functionality to allow communication with public switched telephone network (PSTN) 18.
The use of CDMA modulation techniques as specified by the IS-95 standard makes each RF signal appear as background noise during the processing of any particular RF signal. Making other signals appear as background noise allows transmission of multiple RF signals over the same RF bandwidth. Transmitting multiple signals over the same RF bandwidth increases the frequency reuse of the cellular telephone system, which in turn increases the overall capacity.
To further increase overall system capacity, IS-95 varies the average transmit power of a signal in response to changes in voice activity. The average transmit power is varied in 20 ms increments, either by a reduction in the transmit duty cycle, or an actual transmit power reduction. By varying the average transmit power in response to voice activity, the average total power used by the RF signal to conduct communication is reduced.
Because voice activity is essentially random, however, the total transmit power of an IS-95 compliant base station varies over time in response to changes in voice activity. Thus, when voice activity is low, or few conversations are being conducted, the base station will transmit data at much less than its maximum rate, leaving valuable bandwidth unused.
Furthermore, varying transmit power with voice activity creates a degree of uncertainty as to the total transmit power that will be used at any particular instant. To account for this uncertainty, IS-95 compliant base stations typically transmit at less than the maximum rate to establish reserve transmit power for handling bursts of increased speech activity. Maintaining this reserve, however, also causes the average transmission rate to be less than the maximum transmission rate of which base station is capable.
In any case, transmitting at an average rate that is less than the maximum rate is undesirable, as it does not utilize the available RF bandwidth as efficiently as possible. To increase the usefulness of a CDMA wireless communication system, the present invention is directed towards allowing the average data transmission rate to more closely equal the maximum transmission capacity of a base station 12, and therefore towards increasing the efficiency with which the allocated RF bandwidth is used.
The present invention is a novel method and apparatus for transmitting voice and data in a code division multiple access (CDMA) wireless telecommunications system. In an exemplary embodiment of the invention described herein, a base station transmits voice information at voice transmit power and data at a data transmit power that is equal to a maximum transmit power minus the voice transmit power. In a preferred embodiment of the invention, each base station reports the voice transmit power to a base station controller in 20 ms intervals. The base station controller responds by calculating an available data transmit capacity, and by forwarding data to each base station equal to the available data transmit capacity. Each base station then transmits all voice data at the current voice transmit power, and data at the current data transmission power. If the amount of data received exceeds the capacity provided by the current data transmission power, some data is not transmitted. The base station then notifies the base station controller data was not transmitted, and the base station controller attempts to retransmit the data at a later time.